They are grayer, 35 years after the glory days, and their memories are not as keen. But the engineers who landed Americans on the moon are as enthralled as ever by the romance of space travel. And today some find themselves, to their delight, working once again to send humans to the lunar surface.

Now in their 60s or older, veterans of the Apollo program have been called away from their tennis games, their grandchildren and their puttering to pursue President Bush's goal of returning astronauts to the moon by 2020. Some are serving as part-time consultants to NASA or private companies. Others are working full time on the new moon program. All say the work is a joy, not a burden.

"I'm about to fulfill a dream I've had for 30 years, which is to be a part of getting back to exploration," says former Apollo engineer Leonard Nicholson, 67, now a leader of a Boeing-Northrop Grumman team designing a possible moon ship. "It's a blast for me."

"The excitement is incredible," says Mel Rimer, 67, who is contacting fellow alumni of the lunar-lander program to ask whether they would like to work on the latest model. "Some of the e-mails that we get: 'Can't wait to come back.'"

No human footfall has stirred the moon's fine dust since 1972, when astronaut Eugene Cernan climbed back into his spaceship and declared that humans, "God willing ... shall return, with peace and hope for all mankind."

After that last mission, Apollo team members swallowed their disappointment and found work in other programs. They've been top bosses of the space shuttle fleet, helped run the International Space Station, studied nuclear-powered spaceships. But they have not forgotten the business left unfinished on the moon.

"We had plans (that) ... the moon would become an outpost for experimentation and exploration," says Bob Haslett, 69, who, like Rimer, worked on the lunar lander and is now tracking down old colleagues and advising NASA. "It never happened. So I personally feel good about going back."

Going back should not pose the same challenge as in the 1960s, the Apollo alumni say. More is known about the moon -- for example, that a lander won't sink out of sight into the lunar dust, as was feared back then.

Technological progress will also give today's engineers a leg up. Apollo veterans remember sketching out plans on a drawing board rather than a computer screen, relying on slide rules instead of calculators and standing in line at the painfully slow photocopiers of the day.

To run even a simple computer program, "you had to make punch cards," Haslett recalls. "Then a keypunch operator would punch them in, and God forbid somebody would drop your box of cards and they'd get all out of order."

Yet the plan devised with this rudimentary technology was so good that NASA has decided to reuse it nearly intact. As in the Apollo program that went to the moon in the 1960s and '70s, the next moon vehicle will detach from its rocket to descend to the lunar surface. As in Apollo, the ship will be wingless, and parachutes will cushion its touchdown on Earth.

"We tried hard initially to not make this new (effort) look like Apollo," says John Connolly, a leader of the NASA team that designed the new moon mission. But "we started coming to the realization more and more that the Apollo guys were pretty darned smart."

So smart that NASA Administrator Michael Griffin invited a group of Apollo veterans to Washington several times to review NASA's findings. Griffin called them "the graybeards," says Robert Seamans, 87, who was NASA's deputy administrator for much of the moon program.

Seamans declines to discuss his advice to NASA. Connolly says Seamans outlined the pros and cons of a lunar lander that returns to Earth straight from the moon's surface, rather than having to hook up with its rocket and control systems in lunar orbit. NASA decided that Seamans and his team chose rightly: NASA's new plan, like Apollo, relies on the second option, though studies done in the 1990s favored the first.

Another "graybeard," former Apollo engineer Bob Sieck, 67, used the trips to Washington to argue for simplicity. It's a virtue he learned in part from the 1967 fire that killed three astronauts as they sat in their Apollo capsule on the launch pad. The capsule, as basic as it was, was still so complex that engineers never pinpointed the fire's exact cause, Sieck says.

"That just reinforces that, hey, the simpler you can make these vehicles, the better off you are," Sieck says. "Sure, take advantage of the new technology ... but that doesn't mean you have to make it so complicated that you generate more opportunities for errors."

Rimer says he surprised younger NASA engineers by telling a cautionary story about the first moon landing. It was widely believed that Neil Armstrong nearly ran out of fuel setting his ship down on the lunar surface. Not so, Rimer says. The real problem was a badly designed fuel gauge, which was uncovered months later and is little known today.

"If you try to do it the same way (as on Apollo) ... you get the same problem," Rimer says.

The veterans are enthusiastic about resuming moon flights, which they say the nation was shortsighted to end. But they are also cautious about the chance of success in the 21st century. Today's rocket scientists, they say, may have zippier computers. But they lack some of the crucial assets that made Apollo a success.

"When we needed something, all we had to do was ask," Sieck recalls. "The people that had the checkbook said, 'Fine, it's on its way.'" Whether the USA will spend the estimated $100-billion-plus it will take to return Americans to the moon is a big "if," he says.

Seamans notes another key factor: national resolve.

"Back then, there was a tremendous trauma each time the Soviets put something up," he says. "There was great pressure for NASA to do much more. ... Today, I don't see that."

Despite their skepticism, none of them wants to be left out if the project proceeds.

If the program advances, "I'd love to get involved again. I'd love to do things," Rimer says.